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AJvYcCVxIezErGwJuTfjzq6Pz1m7wF8JUX/7HI0rCY7HZHOusJUe/+JlBPH+/pChF+5udAY6Ubk=@vger.kernel.org X-Gm-Message-State: AOJu0YxLUipAAKogHlUQ0SyXRZNZckSGOv4ZH0ka77BuGguD8Oq4ZXL+ aNmOs8mXsNTjDqJfUqqtYHCEJYGdVQvYxiKTTmHDaWW09nAMldYgVQe/M16OsH3V5/vH0ozu8dV LgQ== X-Google-Smtp-Source: AGHT+IFXhtO/oCrGt8i6oaoAkVXwlRYFse6HqXBKv3/vHJ+bchQ+wvlklzU0zw/eC2aVXwFU/rrZhmvXx58= X-Received: from pfar2.prod.google.com ([2002:a05:6a00:a902:b0:728:b8e3:993f]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a00:2184:b0:725:ef4b:de33 with SMTP id d2e1a72fcca58-72daf88b1b9mr27723623b3a.0.1737479384623; Tue, 21 Jan 2025 09:09:44 -0800 (PST) Date: Tue, 21 Jan 2025 09:09:43 -0800 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250118005552.2626804-1-seanjc@google.com> <20250118005552.2626804-5-seanjc@google.com> Message-ID: Subject: Re: [PATCH 04/10] KVM: x86: Set PVCLOCK_GUEST_STOPPED only for kvmclock, not for Xen PV clock From: Sean Christopherson To: paul@xen.org Cc: Paolo Bonzini , David Woodhouse , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+352e553a86e0d75f5120@syzkaller.appspotmail.com, Paul Durrant , David Woodhouse , Vitaly Kuznetsov Content-Type: text/plain; charset="us-ascii" On Tue, Jan 21, 2025, Paul Durrant wrote: > > --- > > arch/x86/kvm/x86.c | 20 ++++++++++++++------ > > 1 file changed, 14 insertions(+), 6 deletions(-) > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > index d8ee37dd2b57..3c4d210e8a9e 100644 > > --- a/arch/x86/kvm/x86.c > > +++ b/arch/x86/kvm/x86.c > > @@ -3150,11 +3150,6 @@ static void kvm_setup_guest_pvclock(struct kvm_vcpu *v, > > /* retain PVCLOCK_GUEST_STOPPED if set in guest copy */ > > vcpu->hv_clock.flags |= (guest_hv_clock->flags & PVCLOCK_GUEST_STOPPED); > > - if (vcpu->pvclock_set_guest_stopped_request) { > > - vcpu->hv_clock.flags |= PVCLOCK_GUEST_STOPPED; > > - vcpu->pvclock_set_guest_stopped_request = false; > > - } > > - > > memcpy(guest_hv_clock, &vcpu->hv_clock, sizeof(*guest_hv_clock)); > > if (force_tsc_unstable) > > @@ -3264,8 +3259,21 @@ static int kvm_guest_time_update(struct kvm_vcpu *v) > > if (use_master_clock) > > vcpu->hv_clock.flags |= PVCLOCK_TSC_STABLE_BIT; > > - if (vcpu->pv_time.active) > > + if (vcpu->pv_time.active) { > > + /* > > + * GUEST_STOPPED is only supported by kvmclock, and KVM's > > + * historic behavior is to only process the request if kvmclock > > + * is active/enabled. > > + */ > > + if (vcpu->pvclock_set_guest_stopped_request) { > > + vcpu->hv_clock.flags |= PVCLOCK_GUEST_STOPPED; > > + vcpu->pvclock_set_guest_stopped_request = false; > > + } > > kvm_setup_guest_pvclock(v, &vcpu->pv_time, 0, false); > > + > > + vcpu->hv_clock.flags &= ~PVCLOCK_GUEST_STOPPED; > > Is this intentional? The line above your change in kvm_setup_guest_pvclock() > clearly keeps the flag enabled if it already set and, without this patch, I > don't see anything clearing it. Oh, I see what you're getting at. Hrm. Yes, clearing the flag is intentional, otherwise the patch wouldn't do what it claims to do (set PVCLOCK_GUEST_STOPPED only for kvmclock). Swapping the order of this patch and the next patch ("don't bleed ...") doesn't break the cycle because that would result in PVCLOCK_GUEST_STOPPED only being applied to the first active clock (kvmclock). The only way I can think of to fully isolate the changes would be to split this into two patches: (4a) hoist pvclock_set_guest_stopped_request processing into kvm_guest_time_update() and (4b) apply it only to kvmclock, and then make the ordering 4a, 5, 4b, i.e. "hoist", "don't bleed", "only kvmclock". 4a would be quite ugly, because to avoid introducing a functional change, it would need to be: if (vcpu->pv_time.active || vcpu->xen.vcpu_info_cache.active || vcpu->xen.vcpu_time_info_cache.active) { vcpu->hv_clock.flags |= PVCLOCK_GUEST_STOPPED; vcpu->pvclock_set_guest_stopped_request = false; } But it's not the worst intermediate code, so I'm not opposed to going that route. > > + } > > + > > #ifdef CONFIG_KVM_XEN > > if (vcpu->xen.vcpu_info_cache.active) > > kvm_setup_guest_pvclock(v, &vcpu->xen.vcpu_info_cache, >